Planar-shielded meander slow-wave structure

ABSTRACT

A novel slow wave structure capable of ultrabroadband operation includes a meander line of a predetermined length on a layer of insulative material, a first series of spaced metal fingers forming a comb line are attached to the layer and extend in parallel from a front edge of the layer into the facing concave portions of the meander line and a second series of spaced metal fingers forming a comb line are attached to the layer and extend in parallel into the concave portions of the meander line from the opposed back edge of the layer. A metal layer or ground plane is connected to the underside of the layer and both series of metal projecting fingers are placed electrically in common together with the bottom ground plane. In a preferred method of manufacturing of the invention the insulative layer is completely coated or covered with metal on all surfaces and using a photoresist etching process the insulative spaces between the meander lines and the fingers are etched out leaving the desired structure.

United states Patent [.91

Chaffee [54] PLANAR-SHIELDED MEANDER SLOW- [21] Appl. No.: 188,893

[52] U.S. Cl. ..333/31 R, BIS/3.5, 333/84 M [51] Int. Cl. ..H03h 7/30[58] Field of Search ..333/31 A, 84 M; 315/35 [56] References CitedUNITED STATES PATENTS 3,504,222 3/1970 Fukushima ..315/3.5 X

3,610,999 10/1971 Falce ..315/3.5

3,231,780 1/1966 vFeinstein' ..315/3.5

3,670,196 6/1972 Smith ..315/3.5 3,504,223 3/1970 Orr et al. ..315/3.53,594,665 7/1971 Martin et al. 333/31 A 3,610,998. 10/1971 Falce..315/3.5 3,670,197 6/1972 Unger ..3l5/3.5

[ May 29, 1973 Primary Examiner-Rudolph V. Rolinec AssistantExaminer-Saxfield Chatmon, Jr. AltorneyRonald M. Goldman, Alan C. Roseand I Alfred B. Levine [57] ABSTRACT A novel slow wave structure capableof ultrabroadband operation includes a meander line of a predeterminedlength on a layer of insulative material, a first series of spaced metalfingers forming a comb line are attached to the layer and extend inparallel from a front edge of the layer into the facing concave portionsof the meander line and a second series of spaced metal fingers forminga comb line are attached to the layer and extend in parallel into theconcave portions of the meander line from the opposed back edge of thelayer. A metal layer or ground plane is connected to the underside ofthe layer andboth series of metal projecting fingers are placedelectrically in common together with the bottom ground plane. In apreferred method of manufacturing of the invention the insulative layeris completely coated or covered with metal on all surfaces and using aphotoresist etching process the insulative spaces between the meanderlines and the fingers are etched out leaving the desired structure.

8 Claims, 6 Drawing Figures z Sheds-Sheet 1 I FUNDAMENTAL FORWARD MODEFUNDAMENTAL BACKWARD oos I FIELD OF THE INVENTION This invention relatesto slow wave structures and, more particularly, to a shielded slow wavestructure capable of broadband operation.

BACKGROUND OF THE INVENTION Microwave and other types of electromagneticenergy propagatesat a velocity, approximately, equal to the speed oflight. In many known applications it is desired for the microwave energyto be propagated over a predetermined distance at a substantially slowervelocity. For this purpose circuits have heretofore existed which aredenoted appropriately by the terms delay line or slow wave circuit. Inone type, of conventional slow wave circuit the velocity of travel ofelectromagnetic energy between two points is slowed down effectively bycausing the electromagnetic energy to travel in a circuitous path inbetween those two points. One such type of common slow wave circuit isthe meander line, a line of a geometry which in part zigs and zags backand forth laterally in the space between those two given points as itextends over the distance between the two points.

' Typically, slow wave circuits of this type find prime application inboth high frequency microwave'tubes such as the conventional M-typetraveling wave tube and in the high frequency cathode ray tubes. Intraveling wave tubes an electron beam containing electrons traveling ata predetermined velocity, suitably l th the speed of light, is caused tointeract with electromagnetic energy applied to and traveling along theslow wave circuit at approximately the same velocity. Through electronicinteraction the energy in the electron beam is transferred in part tothe propagating electromagnetic energy. And in deflection circuits ofcathode ray tubes having very, very fast rise times the slow wavecircuit provides a means of providing sufficiently and prevent thefields from jumping or coupling berapid deflection to permit display ofvery transient signals typically on the order of 0.1-1 nanoseconds.

In each application a slow wave circuit is best if it has a largefrequency bandwidth or, as otherwise termed, is capable of broadbandoperation; that is, the velocity with which a microwave signalpropagates effectively along the slow wave circuit should remainessentially constant regardless of the frequency of that signal.Typically a slow wave circuit which is capable of providing such uniformvelocity of travel over a l-octave range of frequencies, such as from 2to 4 Gigahertz, is considered to be broadband and in the past the helixhas been considered the best available broadband slow wave circuit.

The meander line configuration of slow wave structure, referred topreviously, in itself has limited bandwidth characteristics,particularly at lower frequency ranges relative to the midband frequencyat which the line is designed to be used. It is possible for theelectromagnetic flelds to jump" across or bridge turns rather thanfollow the circuitous path of the meander line. In jumping turns theelectromagnetic field takes a shortcut over a portion of the definedtransmission path and therefore travels the length of the meander lineat a faster effective velocity than is the case in which the meanderpath is followed. This jumping or coupling from turn to turn in themeander line is largely dependent upon the frequency of the propagatingsignal and the delay of the line varies. Thus in that low frequencyregion the line is not broadband and does not provide a uniform delay.

To prevent the electromagnetic signal from bridging the turns of themeander. line, various forms of shielding have been proposed. In suchconstruction an electrically conductive member is interposed between theturns in the meander line and connected electrically to ground potentialso as to shield one turn from another tween turns. With this typeof'line a metal ground plane was placed underlying the line and theshielding members were projected upward from this metal base between theturns of the meander line. Pat. U.S. No. 3,504,222 is an example of suchconstruction. That proposed construction is necessarily difficult toconstruct because it involves a series of three-dimensional teeth whichproject upward from the ground plane into the space between the turns.In particular, this difficultyis understood in that at a frequency ofabout 5 Gigahertz (GHz) the turn-to-tum spacing is on the order of 0.040inch and must be made to a tolerance of: 0.001 inch.

OBJECTS OF THE INVENTION BRIEF SUMMARY OF THE INVENTION In accordancewith the invention,'an elongated thin layer of electricallynonconductive or other dielectric material, suitably aluminum oxideceramic or boron nitride ceramic, serves as a base or supportingsubstrate. A pair of electrically conductive comb-like lines, suitablyof gold chromium, are oriented geometrically with their teeth projectingin opposite directions on the plane of the base surface with the combteeth interdigitally arranged and spaced on the top surface of the baseand with some spacing between the teeth of one comb line and any portionof the teeth of the opposed comb-like line. An electrical conductor isspaced from and meanders about and in between those projectinginterdigitally arranged .teeth and is coupled, suitably bonded, tothe'base surface. This conductor is of the geometry of a meander line. Ametal layer is coupled, suitably bonded, to the bottom or undersidesurface of the base and provides a ground plane. And each of the callyin common.

In accordance with the preferred method of construction of my invention,an elongated layer of insulative material is first coated, suitably byconventional deposition or metallizing techniques, on all surfaces witha thin metal layer. A conventional photoresist process is used to maskthe outline of the desired configuration of comb and meander lines on atop surface of this coated insulative material and the space betweeneach of the metal members is thereupon etched out to expose thedielectric base and provide the desired configuration.

The foregoing objects and advantages of my invention, together withequivalents and substitutions for the elements thereof and thefunctional relationship between the elements, are better understood froma consideration of a preferred embodiment of my invention as describedin the detailed description which follows taken together with theillustrations of the drawings.

FIG. 1 illustrates one embodiment of the slow wave structure of theinvention;

FIG. 2 illustrates schematically a top and side view of the slow wavestructure of FIG. 1;

FIG. 4 illustrates the relationship between the elements comprising theshield and meander line of FIG.

FIG. 3 illustrates the m-B characteristics of the line;

FIG. 5 illustrates the normalized phase velocity characteristic of theembodiment of the invention; and

FIG. 6 illustrates the impedance characteristics of the embodiment ofFIG. 1 of the invention.

DETAILED DESCRIPTION OF THE INVENTION The preferred embodiment of theslow wave structure in FIG. 1 includes a thin elongated rectangular slabor wafer base, which is, by way of specific example, approximately l.l50inches in length, 0.360 inches in width, and 0.040 inches thick indimension. This base material is electrically nonconductive dielectricmaterial, suitably boron nitride, and is visible in the embodiment ofFIG. 1 at the locations 3 on the top surface of the base and location 5in cutaway portion 4. A series of straight spaced parallel metalprojecting fingers or teeth, 7a, 7b, 7c, 7d and 7e, extend along theupper surface from a metal bar 9 which extends parallel along the 'backedge of the base. Each of the fingers 7a through 7e is seen to projectfrom bar 9 perpendicularly thereto from the back edge or side 11 of thebase. A plurality of like straight spaced parallel metal fingers orteeth, 15a, 15b, 15c, 15d, 15c and 15f, are attached to the base andextend along the upper surface from a essentially interdigitallyarranged relative to one another. Suitably the distance center-to-centerbetween the teeth 15 and 7 in a specific example is on the order of0.040 inch and the teeth-to-teeth center-to-center spacing of any onecomb line is approximately 0.080 inch for operation up to a frequency of5 Gigahertz (GHz). In one specific example of the preferred embodimentat that frequency the teeth 7 and 15 are approximately 0.270 inch inlength, 0.010 inch in width, and 0.001 inch in thickness.

An additional electrical conductor 17, suitably of gold-chromiummaterial, is attached or coupled to the dielectric base material.Conductor l7 follows a path in the figure which winds or zig zags, asvariously may be termed, in between the projecting teeth 7 and 15 of thelines without contact therebetween. Starting from the end 20 locatednear the back end of bar 13, line 17 extends parallel and spaced fromtooth 15a to within a predetermined distance of bar 9, wherein the linethereupon extends parallel to bar 9 to pass the tip of tooth 15a. Line17 then reverses direction and extends parallel and in between teeth 15dand 7a to a predetermined distance of bar 13. In the space between thetip of tooth 17a and bar 13, line 17 extends toward the front edge ofthe base parallel to bar 13 and extends past the tip of tooth 17a to aposition in between teeth 17a and 15b, whereupon line 17 is againreversed in direction and extends in between and parallel to teeth 7aand 15b to within a predetermined distance of bar 9. At this point line17 again extends parallel to bar 9 toward front bar 13 to apredetermined distance in between teeth 15b and 7b. The furtherdescription of the path taken by line 17 to its end 21 is apparent fromFIG. 1. By such a circuitous course or geometry, line 17 meanders or zigzags, as is variously termed, back and forth between front edge and backedge of the layer extending forward in steps between the points ofbeginning 20 and end 21 of this line. A line of this geometry isconventionally described in the prior art as a meander line. As orientedin the embodiment the teeth of the comb lines extend into theconfronting concave portions of the meander line from each side. In thespecific example cited previously, the, center-to-center spacing metalbar 13 which extends parallel along the front.

edge of the base. Each of these teeth 15a through 15f is seen to projectperpendicularly from bar 13 from the front side of the base. The teeth15 are laterally displaced from the teeth 7 so as to fit in the spacebetween teeth in the oppositely arranged structure without contact andthe tips of the teeth extending in one direction do not contact the barsattached to the teeth that extend from the other direction. Suitably bar9 and projecting teeth 7 form a comb-like structure conventional in theprior art and known as a comb line. Likewise, bar 13 and projectingteeth 15 also form appropriately a comb-like structure or comb line. Itis seen in the figure that the teeth of comb l5 fit in between and arespaced from the oppositely directed teeth 7 of the opposed comb, and inthis relationship the comb lines are between the meander line '17 andadjacent teeth is on the order of 0.020 inch and line 17 isapproximately 0.010 inch in width, the same as the width of each of thecomb line teeth, and is approximately 0.001 inchin thickness. Theoverall length of line 17 is approximately 0.270 inch or, expressed inwavelengths, 54 X at the upper frequency of 5 GHz. v

A metal layer 23, as is visible in cutout portion 4, covers the entirebottom of the dielectric base. Each of the side edges 11, 12, the backedge, not visible in this figure, as well as the left side edge 18 andportions of the right and left side surfaces, are covered entirely 'neednot cover the bottom surface entirely and can include openings in lesspreferred embodiments. And the electrical paths between the ground planeand the comb lines can be made by electrical wires as an obviousalternative to the plating of edge surfaces for less preferredembodiments.

Line 17 terminates at its front and back ends 21 and 20 just short ofthe comb lines. Electrical leads can be connected to line 17 at ends 20and 21 as input and output connections to other circuitry and a groundlead may be connected to the metal layer 23. However it is noted thatthe ends of line 17 can alternatively be connected in common with thecomb lines. Inasmuch as the length of the line. is many wavelengths longat the microwave frequencies coupled to the meander line, the linepossesses a definite impedance and hence the coupling in this mannerdoes not act as an electrical short circuit. In such a modificationexternal signal connection to the line 17 is accomplished by anavailable non-contacting coupling, such as inductive or capacitive.

FIG. 2 schematically shows the slow wave structure of the invention, theends 20' and 21' of the meander line 17- which meanders between teethand 7 of the interdigitally arranged comb line, and electrical input andoutput connecting leads coupled to the input and output ends of line 17represented by the dashed lines 25 and 27.

The (0-3 diagram of FIG. 3 shows the theoretical relationship,calculated for the embodiment of FIG. 1, between the frequency and thephase shift of the fundamental mode. As is conventional, this type ofdiagram is useful for design purposes since phase and group velocity canbe determined by inspection. It can be seen, for example, that the phasevelocity of the fundamental forward mode is nearly constant while thefundamental backward mode is very dispersive.

The fundamental backward wave mode propagates between (.0 and 01 Thesefrequencies to, and 00 are controlled by the distance, W, to the groundplane, and the bandwidth, (lily-(0 can be made small by close groundplane spacings. For frequencies less than to, the

structure propagates only the mode.

The sectional drawing of the embodiment of FIG. 1 as is illustrated inFIG. 4 represents those dimensions of the circuit, pitch of the line, Pand L, a factor related to the length of the line, which determineessentially the relationship between to and B in the curve of FIG. 3.Thus to decrease the upper frequency of operation of the line, L is madegreater in length. By increasing P, the phase velocity increases. As isapparent, many other variations in these parameters are possible. FIG. 5graphically illustrates the normalized phase velocity vs frequency,thedispersion characteristics,

fundamental forward for the embodiment of FIG. 1. It is seen that thedispersion characteristic is essentially nondispersive from 0 (d.c.) to5 6112. By theoretical calculation at 1.5 GHz derived from the followingmathematical equation. The

equation characterizes the slow wave structure for that particularexample in which the width dimension of the meander line is the same asthe width dimension of the teeth of the comb line, which were both 0.0!inch in the example given, and is not valid for other possiblevariations of my invention in which the width of the two differs:

where k m/c v m 21rf, wherefis frequency in Gl-Iz c velocity of light Y4Y[0+(31r/2)l where1r=3.I4...

0 Bp, where B is the variable phase and p is the pitch and where q isthe dielectric gap distance between the comb line teeth and the meanderline a is q/p previously defined and 1(W,a)= 2 (-1) coth viding by c,the velocity of light, to obtain the normalized dispersive curve of FIG.4.

FIG. 6 graphically illustrates the generalized coupling impedance of theslow wave circuit of my invention. In this the coupling impedance K (0.)W is plotted against frequency. The factor er is dimensionless andrepresents the relative dielectric constant or permittivity of the basematerial. Thus obtaining the generalized" impedance is 29 ohms, where erfor boron nitride is 4.1. It is noted that the coupling impedance isless than 100 ohms in the range of O to 5 Gl-Iz and this is consideredlow impedance.

There are numerous ways to form the essentially thin flattwo-dimensional structure of the slow wave circuit. It is preferred toform the slow wave structure by taking the dielectric base material ofthe appropriate dimension as determined from the foregoingconsiderations and covering all surfaces suitably with a coating of approximately 0.001 inch of chromium-gold layers by conventional thin filmdeposition techniques. In this a small flash of chromium is firstdeposited followed by depositing the gold. The chromium serves to bondthe gold to the ceramic base of boron nitride. A photoresist material isthen applied to all surfaces and the upper surface is then exposed to aphotographic or masked image to establish the pattern of comb lines andmeander lines as illustrated in FIG. 1. Thereupon the formed structureis placed in a conventional etchant bath which etches out the goldchromium material in the places where the photoresist material was notexposed and this etching is carried through until the dielectric surfaceunderlying the metal is removed leaving the pattern of insulative spacesbetween the metal lines as illustrated in FIG. 1. Thereupon thephotoresist material is removed and the slow wave circuit isadditionally cleaned.

In the practice of the invention what is derived is a precise and yetsimple to construct configuration which in the example of the preferredembodiment possesses an essentially dispersionless characteristicbetween and Gigahertz. Obviously, this form of packaging permits manysuch structures to be stacked one upon another with suitable insulatorspacing therebetween in some desired applications. Also, inasmuch asmachining and other crude methods of fabrication are avoided entirely,and accurate photographic techniques are used in its construction, amore reliable and at the same time less expensive structure is obtained.

The foregoing details of the preferred embodiment of my invention arepresented to enable the reader to clearly understand the nature of theinvention and how to make and use the invention as required by thepatent laws and not by way of limitation to my invention. It isunderstood that my invention is not to be limited to the detailsillustrated in the preferred embodiments inasmuch as many equivalentsand substitutions to the foregoing disclosed elements suggest themselvesto one of ordinary skill in the art upon reading this specification.Accordingly, my invention is to be broadly construed within the spiritand scope of the appended claims.

What I claim is:

1. A slow wave structure comprising:

a continuous layer of dielectric material of a first predeterminedthickness;

a first metal comb-like structure, said comb-like structure comprising aplurality of flat spaced projecting teeth connected to a common flatbar-like member;

a second metal comb-like structure, said second comb-like structurecomprising a plurality of flat spaced projecting teeth projecting from acommon flat bar-like member;

said first and second comb-like structures having a second predeterminedthickness less than said first predetermined thickness;

each of said first and second comb-like structures being attached to anupper surface of said insulative layer'with the teeth of said combsprojecting in opposed directions along said upper surface and beinginterdigitally arranged, leaving space between said adjoining teeth andbetween the tips of said teeth and said bar to define a meander-shapedinsulative space therebetween;

a ground plane conductive metal layer attached to and covering thebottom surface of said insulative layer; a conductive metal coating onone side wall of said layer for electrically connecting said groundplane layer in common with said first comb-like structure and aconductive metal coating on another side wall of said layer forelectrically connecting said ground plane in common with said secondcomb-like structure; and

a meander-shaped electrically conductive flat metal strip applied tosaid upper surface and located in said meander-shaped insulative space.

2. The invention as defined in claim 1 wherein each of said first andsecond comb structures and said meander-shaped strip' comprise themetals gold and chromium, and wherein said insulative layer comprisesboron nitride.

3. The invention in a slow wave structure comprising: an elongated slabof insulative material having a first predetermined thickness, a coatingof electrically conductive material of a second predetermined thicknessless than said first predetermined thickness covering all surfaces ofsaid insulative slab except a first exposed insulator portion on saidupper surface, said exposed insulator portion comprising a geometry of ameander line of a predetermined length and a predetermined pitch and apredetermined width, and meander line shaped electrical conductor meanslocated within said exposed portion and attached to said slab surfaceand spaced from said first metal coating, said electrical con-v ductormeans having a width less than said predetermined width and the samepredetermined pitch whereby shielding fingers are formed between theturns of the meander line which shielding fingers do not extend throughthe slab to the bottom metal surface but are connected from one end tothe bottom metal layer via the metal coating on the sides of the slab ofinsulative material.

4. The invention as defined in claim 3 including an input terminal meansconnected to one end of said meander line and output terminal meansconnected to the other end of said meander line.

5. The invention as defined in claim 3 further including firstelectrical conductor means joining one end of said meander line to anadjacent portion of coating, and second conductor means joining theremaining end of said meander line to an adjacent portion of coating.

6. The invention as defined in claim 3 wherein said insulative slabcomprises a fired ceramic dielectric material and wherein said firstmetal coating comprises gold chromium and wherein said meander linecomprises the metal gold chromium.

7. A slow wave structure comprising a thin metal meander line, arelatively thick layer of insulative material, said meander line beingcoupled to and traversing over a top surface of said layer of insulativematerial; a plurality of thin metal fingers spaced from one another andprojecting from a thin metal bar perpendicular to said fingers to form athin comb-like structure on said meander line within a correspondingconcave portion of each meander in said meander line and spacedtherefrom; an electrically conductive metal ground plane attached to andcovering the bottom surface of said layer of insulative material; andthin metal electrical conductor means attached to the sides of saidlayer for connecting each of said first and second bars of saidcomb-like structures electrically in common with said ground plane.

8. The invention as defined in claim 7 wherein said last-named meanscomprises a first thin metal layer coupled to and covering a sidesurface of said layer adjoining said top and bottom surfaces of saidlayer for electrically connecting said metal bar of said first comb-likestructure to said metal ground plane and a second thin metal layercoupled to and covering another-side surface of said layer adjoiningsaid top and bottom surfaces of said layer for electrically connectingsaid metal bar of said second comb-like structure to said metal groundplane.

1. A slow wave structure comprising: a continuous layer of dielectricmaterial of a first predetermined thickness; a first metal comb-likestructure, said comb-like structure comprising a plurality of flatspaced projecting teeth connected to a common flat bar-like member; asecond metal comb-like structure, said second comb-like structurecomprising a plurality of flat spaced projecting teeth projecting from acommon flat bar-like member; said first and second comb-like structureshaving a second predetermined thickness less than said firstpredetermined thickness; each of said first and second comb-likestructures being attached to an upper surface of said insulative layerwith the teeth of said combs projecting in opposed directions along saidupper surface and being interdigitally arranged, leaving space betweensaid adjoining teeth and between the tips of said teeth and said bar todefine a meander-shaped insulative space therebetween; a ground planeconductive metal layer attached to and covering the bottom surface ofsaid insulative layer; a conductive metal coating on one side wall ofsaid layer for electrically connecting said ground plane layer in commonwith said first comb-like structure and a conductive metal coating onanother side wall of said layer for electrically connecting said groundplane in common with said second comb-like structure; and ameander-shaped electrically conductive flat metal strip applied to saidupper surface and located in said meandershaped insulative space.
 2. Theinvention as defined in claim 1 wherein each of said first and secondcomb structures and said meander-shaped strip comprise the metals goldand chromium, and wherein said insulative layer comprises boron nitride.3. The invention in a slow wave structure comprising: an elongated slabof insulative material having a first predetermined thickness, a coatingof electrically conductive material of a second predetermined thicknessless than said first predetermined thickness covering all surfaces ofsaid insulative slab except a first exposed insulator portion on saidupper surface, said exposed insulator portion comprising a geometry of ameander line of a predetermined length and a predetermined pitch and apredetermined width, and meander line shaped electrical conductor meanslocated within said exposed portion and attached to said slab surfaceand spaced from said first metal coating, said electrical conductormeans having a width less than said predetermined width and the samepredetermined pitch whereby shielding fingers are formed between theturns of the meander line which shielding fingers do not extend throughthe slab to the bottom metal surface but are connected from one end tothe bottom metal layer via the metal coating on the sides of the slab ofinsulative material.
 4. The invention as defined in claim 3 including aninput terminal means connected to one end of said meander line andoutput terminal means connected to the other end of said meander line.5. The invention as defined in claim 3 further including firstelectrical conductor means joining one end of said meander line to anadjacent portion of coating, and second conductor means joining Theremaining end of said meander line to an adjacent portion of coating. 6.The invention as defined in claim 3 wherein said insulative slabcomprises a fired ceramic dielectric material and wherein said firstmetal coating comprises gold chromium and wherein said meander linecomprises the metal gold chromium.
 7. A slow wave structure comprising athin metal meander line, a relatively thick layer of insulativematerial, said meander line being coupled to and traversing over a topsurface of said layer of insulative material; a plurality of thin metalfingers spaced from one another and projecting from a thin metal barperpendicular to said fingers to form a thin comb-like structure on thetop surface of said layer; said metal fingers being bonded to saidinsulative material and protruding from one side of said meander linewithin a corresponding concave portion of each meander in said meanderline and spaced therefrom; a second plurality of metal fingers spacedfrom one another and projecting from a thin metal bar perpendicular tosaid fingers to form a thin comb-like structure on the top surface ofsaid insulative layer; said metal fingers being bonded to saidinsulative material and protruding from the other side of said meanderline within a corresponding concave portion of each meander in saidmeander line and spaced therefrom; an electrically conductive metalground plane attached to and covering the bottom surface of said layerof insulative material; and thin metal electrical conductor meansattached to the sides of said layer for connecting each of said firstand second bars of said comb-like structures electrically in common withsaid ground plane.
 8. The invention as defined in claim 7 wherein saidlast-named means comprises a first thin metal layer coupled to andcovering a side surface of said layer adjoining said top and bottomsurfaces of said layer for electrically connecting said metal bar ofsaid first comb-like structure to said metal ground plane and a secondthin metal layer coupled to and covering another side surface of saidlayer adjoining said top and bottom surfaces of said layer forelectrically connecting said metal bar of said second comb-likestructure to said metal ground plane.